Asteroid Deflection: The Nuclear Option

by Paul Gilster on March 22, 2007

NASA’s March report to Congress on deflecting Near-Earth Objects offers some startling assessments. Specifically, the report says this: “Nuclear standoff explosions are assessed to be 10-100 times more effective than the non-nuclear alternatives analyzed in this study. Other techniques involving the surface or subsurface use of nuclear explosives may be more efficient, but they run an increased risk of fracturing the target NEO. They also carry higher development and operations risks.”

Fair enough re setting off a nuke on the surface of an asteroid. But aren’t we jumping the gun on other nuclear options when alternatives seem available? That’s certainly the view of Rusty Schweickart, founder of the B612 Foundation, which is all about spreading the word on the threat these objects may pose to Earth. Alan Boyle discussed these matters with Schweickart in a recent post, from which this on the non-nuclear option:

Schweickart argues that the so-called “nuclear standoff” option should be used only as a last resort. He contends that 98 percent of the potential threats can be mitigated by using less extreme measures. For example, he favors the development of a “gravity tractor” – a spacecraft that would hover near an asteroid for years at a time, using subtle gravitational attraction to draw the space rock out of a worrisome path.

To kick it up a notch, Schweickart said a threatening NEO could first be hit with a kinetic impactor – say, a scaled-up version of the Deep Impact bullet that hit Comet Tempel 1 back in 2005 – and then the orbital track could be fine-tuned using the tractor. Navigational sensors aboard the tractor would check to make sure the NEO was on a completely safe path.

“This combination is obviously the way to go,” he said.

We’ve got to get this issue straightened out, because we still don’t know how much of a threat really exists from these objects. That makes the NEO hunt a key part of future space strategy. Schweickart discusses an infrared telescope in an inner-Solar System orbit as one way to get the job done, but read the rest of Boyle’s post for the details, and note the continuing analysis of 99942 Apophis, whose orbital wanderings may ratchet up public awareness of potential impactors once again.

I like creative approaches like Rusty’s gravity tractor, but the viscerial antipathy to various nuclear options expressed in the past by Sagan and by others today smacks of ideology, not science.

What we can all agree on is the need to interact with several NEOs so that we understand their baseline structures and the variations. That data should help design the most effective mitigation strategies, by whatever mechanisms we can practically implement. No options should be off the thinking table for ideological reasons.

I just posted the following at Alan Boyle’s site. I’ve posted on the topic here, before; but it seems appropriate to reiterate…

There’s an approach which would suit some asteroids, given some proto-mining apparatus. I haven’t seen any actual study of it, but it seems worth investigating.

Most asteroids should have some rotation; in fact, there’s recent evidence for a solar radiation driven spin-up mechanism. There will be a distance from the axis of rotation at which centrifugal force exceeds the asteroid’s gravitation.

Anchor a tether, initially with an explosive piton fired from beyond the radius; let it reel out to the radius times two or three.

Fasten to the tether a bucket chain or conveyor belt in the form of a closed loop.

Place on the asteroid surface a crushing and pulverising mechanism, with an initial energy source – 10-100 kW should do as a minimum.

The conveyor belt is ready to operate as a siphon. Prime it, by loading the bottom buckets with dust, and hauling them up to and past the radius. As a bucket passes the radius, centrifugal force will make it pull outward on the tether. If the bucket is kept closed until it reaches a multiple of the radius, the dust content will add to the outward pull.

At a sufficient multiple of the radius, the dust is released, and moves off at a tangent. As operation continues, a disk of dust spreads out from the asteroid.

Once the system is running, the outward pull can drive the belt. In fact, with some gearing or an alternator, it can drive the pulveriser too.

Solar radiation pressure acts on the dust, gradually pushing it into a comet-like plume away from the Sun. The plume is a “gravity tug” pulling in this one direction; but one direction is all we need, to divert the asteroid. The cross-section of the plume will vary from circular to narrowly elliptical, depending on the orientation of the spin axis to the solar radial direction.

For every kilogram per second of dust added to the disk, 86.4 tonnes will be added per day. This compares to the 20 tonnes suggested for an ion-propelled gravity tug. The gravitational effect of the dust will be greater if it is released shortly above the centrifugal radius (because it won’t drift far), but the drawing power of the tether will be greater if it is released at a greater multiple of the radius; this is a tradeoff, but at worst it means we sacrifice some dust.

This, I suggest, is an asteroid deflection scheme that Leonardo da Vinci could have proposed. The failure points are all mechanical, i.e. the belt or the pulveriser might get stuck. That would be a show-stopper; but it should also be a very typical exercise for contemporary astro-robotics students, especially if they have space mining ambitions.

(Mar 19, 2007) Space lasers that zap away rogue asteroids sound like the premise for a 1980s video game. But researchers say the technique could one day be used to detect – and deflect – asteroids found to be barrelling towards Earth. Previously, researchers have proposed several methods to save Earth from an asteroid impact. These include blowing it up with a nuclear bomb or putting a spacecraft beside it so the craft’s gravity could tug the asteroid off course.

I have already seen such comments on another site in the past few days. It is generally par for the course.

I don’t have any “answers” to the situation, per se, but I do know that automatically ruling out possible answers, such as nuclear weapons — which used in this manner are PEACEFUL I might add — is a ridiculous response. As philw correctly points out such statements are indeed “ideology” and definitely not science.

I’d also point out that Bill Weinburg’s site says on their mission statement that, “We endeavor to expose the corporate agendas behind the new military interventions, and to find pro-autonomy, anti-militarist voices we can support in the countries under imperialist assault. We especially seek to loan solidarity to land-rooted, stateless, and indigenous peoples—the “Fourth World.”

That site brought this issue up as a complex conspiracy theory wherein Rusty the Astronaut is advocating nukes, whereas the article above states that he is examining the “non-nuclear” options. So, somewhere there is a middle road on this subject and folks that are throwing out one option as “bad” and suggesting only another option is “good” are probably wrong.

The Asteroid threat is sincerely real, and we’re going to be the species that can stop it from becoming real, assuming we pay attention to it, rather than ignore it, pooh-pooh the situation, or incorrectly assume it is a “political problem with” whatever administration happens to be in charge at the moment.

The dinosaurs probably had similar debates and they ended
up waiting too long to do anything about it before the space
rock hit.

My first objection concerning the nuclear option is that if the
space rock in question is too “brittle” as some astronomers
conjecture about many planetoids, the NEO impactor could
break apart from the nuclear detonation and become an
even bigger threat to Earth.

I would prefer the option to steer the NEO out of the threat
zone so we could then study it and mine it for minerals if
need be. Or hollow it out for a Space Ark.

Anyone who really questions the nuclear option in the event
that an NEO is going to hit Earth and cause mass destruction
and there are no other alternatives in time makes me wonder
if they really have the best interests of the human species
(and the other Earth life) at heart. I can see some eco
fundies who would love to have our civilization brought to its
knees just to save the wildlife. There are less drastic options
for all situations concerned.

As Carl Sagan once said, threatening NEOs may be nature’s
way of getting the intelligent life forms on every planet to
develop space travel and get off or face the consequences.
A bit drastic too, but at least it would get us out there. :^)

Certainly a threat to the species demands more than playing our tired political games?

NEOs range in kind and threat. No one solution is best for all. Best to learn more, quickly, and stand ready to use the most appropriate.

Even the early calculations–dating back to MIT 35 years ago, and that I and Wm. Rotsler then did for SHIVA DESCENDING in 1979 (and still in print)–show that there are several threat classes (in NEO mass and time to impact) that require nuclear warheads. Unlike the pretty bucket brigade idea–which is clever, and should be developed, not least for mining applications in future–we have just about everything we need to deploy a nuclear solution. At least they’re thrifty, though not PC.

gregory and phil correct me if i am wrong but i think that i have read and enjoyed shiva decending afew years ago! enjoyed it. and gregory i have to say thank you at this very time as luck would have it i am reading your book beyond infinity,it is a virtual fund of ideas and i am richly enjoying it! i see you have co authored a book with another of my favorite sf authors arthur c clarke and i plan to soon read that one too! thanks to one and all your friend george

The use of fission-fusion devices would be situationally dependent. Of course some earlier, less extreme method would be preferred. Ideally the method would result in a valuable source of material in high Earth orbit waiting for us to utilize it at our liesure. Conversely if there’s no early option then nuke the bloody thing. I’ll take my chances with a shotgun blast of bird shot versus a .418 to the chest.

The idea that nukes can be effective shouldn’t be a big surprise. Nukes have, what, 1000x more energy per unit mass than other options? I personally have no problem with peaceful uses of nukes, as long as the environemental impact study is solid. There was a proposal to use nukes to dig another canal, like the Panama Canal. The explosions would be underground. I’ve also worked at a nuke power plant. If we had a sane plan for what to do with the spent fuel, it even seems like a good idea.

If one could reduce a space rock to dust (with a nuke), and that dust hit the Earth’s atmosphere over a wide area, one would expect heating (the energy has to go somewhere), and there might be a sonic shock, but would that make it to the ground? Of course, bigger chunks would make it.

I would prefer the option to steer the NEO out of the threat zone so we could then study it and mine it for minerals if need be. Or hollow it out for a Space Ark.

The ESA dual mission Observer/Impactor scenario is the best possible option in the near future. Add a SMART-1 style ion engine and you have the opportunity to look at the Gravitational Tractor as well. In the longer term, it’s pretty clear that nobody wants the sort of militarisation of space which the nuclear option demands, and that ‘slow but steady’ solutions will sort out the vast majority of predicted threats.

Aye, and there’s the rub: prediction.

We need a space observatory, and Rusty Schweickart has clearly identified the need for it to be in a more-or-less Venus orbit to pick up on Earth-crossers. Without a predictive survey we’re reduced to Project Icarus and all that it implies.

Most people have the wrong idea about nuclear weapons based asteroid deflection. In space, a nuclear weapon is really more of an extremely powerful x-ray source. It’s possible to use that x-ray source to ablate a patch of asteroid and create a substantial amount of thrust. Even so, such techniques would require many years of advance notice to be effective. More than likely, the most heavily used asteroid deflection schemes will be those that are more subtle, because it’s cheaper and less risky, and because it’s inevitable that we’ll catalog nearly all potential impactors within the near future.

However, what’s interesting about asteroid deflection is that, much like nuclear fusion as a power source, it is still such a blue sky research area that we do not really have a good understanding of the value of various solutions. There are a wide range of potential solutions being put forward and undoubtedly the usefulness, practicality, and efficiency of these different schemes span a wide margin. But we don’t have a good sense of where they sit in that range, which are the best and which are the worst. That’s a strong indication of our ignorance in these areas, an ignorance that desperately needs remedy.

only nuclear weapons have enough energy to significantly effect the motion
or structure of an asteroid or comet . Even a small asteroid usually masses
millions of tons . Applying newtons second law F= MA it should be self
evident that asteroid deflection, or asteroid destruction requires nuclear explosions. The proposed non nuclear options will not work, and are driven by the extreme far left political ideaology of their advocates and not by science .
tim

NUCLEAR ASTROPHYSICS 1957-2007: BEYOND THE FIRST
50 YEARS (FEATURED EVENTS)

On July 23 to 27, a symposium will be held at Caltech to commemorate
the 50th anniversary of the birth of the field of nuclear
astrophysics. Presenters including distinguished scientists from
around the world will review the progress in the field and discuss
prospects for future advances. For details and registration
(required), go to http://www.NA2007.caltech.edu1.

“New scientific findings suggest that a large comet may have exploded
over North America 12,900 years ago, explaining riddles that scientists have
wrestled with for decades, including an abrupt cooling of much of the planet
and the extinction of large mammals.”

Those who think we can mine asteroids or get them into an earth orbit are woefully misinformed and ignorant of the basic requirements, energy-wise, Even if we got an asteroid into earth orbit, getting ore or refined metal to the surface safely would cost many times more than any ore or gem would be worth…

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

On Comments

Centauri Dreams publishes selected comments on the articles under discussion here. Among the criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations.